import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int Px = Integer.parseInt(sc.next());
        int Py = Integer.parseInt(sc.next());
        int Qx = Integer.parseInt(sc.next());
        int Qy = Integer.parseInt(sc.next());


        System.out.println((Math.abs(Px-Qx) + Math.abs(Py-Qy) / 2.0));
    }
}